Novel fingermark detection techniques using upconverters with anti-stokes luminescence
- Publication Type:
- Thesis
- Issue Date:
- 2012
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Fingerprinting is a mainstay of forensic science and has been used in crime
investigation for more than one hundred years. However, most fingermarks found at
a crime scene are latent; they may become visible through development and
enhancement. Among all the fingermark development techniques, conventional
luminescence methods are routinely employed, with the advantages of being both
sensitive and selective on non-luminescent substrates (i.e., providing high contrast in
developed marks).
Anti-Stokes luminescence or upconversion is an optical process of converting
long-wavelength radiation into a shorter-wavelength emission, which is contrary to
conventional Stokes luminescence. Upconversion mainly exits in rare-earth
complexes and upconversion materials are referred to as upconverters.
Commercially-available upconverters have been widely employed in security inks
and biolabels.
Upconversion is unusual in both natural surfaces and in consumer products. If the
upconverters are applied for fingermark detection and show selective affinity to
fingermark materials, theoretically the strong luminescence of the upconverters can
be visualised on fingermarks as bright regions on a totally dark background. This
means that fingermark detection techniques using upconverters has the potential to
eliminate interference from background printing and luminescence.
This thesis begins with the review of luminescence-based fingermark detection
techniques and previous research on the application of upconverters for fingermark
detection. The previous research showed that upconverters have an affinity for
fingermark residues and are effective for fingermark detection.
Chapter 2 describes issues with respect to imaging the upconversion
luminescence. Of the options tested, a 980 nm laser pointer with 700 mW output
proved to be the most suitable light source for the excitation of the upconversion
luminescence. Long exposure times were needed to record the upconversion
luminescence. A Rofin Poliview fitted with a 555 nm band-pass barrier filter was
found to be a suitable recording system.
Chapter 3 investigates the application of the NaYF4:Er,Yb upconverter powder
for latent fingermark detection on non-porous and semi-porous surfaces. The
NaYF4:Er,Yb powder showed selective affinity to fingermark materials and the dry
powdering method proved to be better than the suspension method. The upconverter
powder showed strong luminescence when illuminated with 980 nm wavelength
laser light and the developed fingermarks presented clear ridges with high contrast.
A near-infrared laser diode and laser pointer are both effective light sources when
used in conjunction with a 555 nm band-pass filter to block the IR light. In actual
imaging, the fingermark substrate is still visible to some extent under long exposure
times, but the interference is reduced compared to what is observed with
conventional luminescence imaging and the fingermark detail is clear. In summary,
the NaYF4:Er,Yb powder can be used to detect fingermarks on various difficult
surfaces that exhibit interfering background luminescence when using conventional
luminescence techniques.
Chapter 4 investigates the application of another type of upconverter powder,
YVO4:Er,Yb, for fingermark detection on non-porous and semi-porous surfaces. The
YVO4:Er,Yb powder proved to be effective for latent fingermark development when
used as a dry powder or as a suspension, with the former generally presenting the
better result. The YVO4:Er,Yb powder also showed selective affinity to fingermark
residues on most surfaces and the developed fingermarks presented clear ridges
against a clear background. The upconverter powder showed strong luminescence
when illuminated with 980 nm wavelength laser light but was slightly less visually
luminescent than the NaYF4:Er,Yb powder. Both a laser diode and pointer are
effective light sources when used in conjunction with a 555 nm band-pass filter to
block the infrared light. As before, the fingermark substrate was visible to some
extent in the upconversion luminescence mode with long exposure times, but the
interference was reduced compared to that observed using conventional
luminescence imaging. Clear fingermark detail was observed. In summary, the
YVO4:Er,Yb powder can be used to detect fingermarks on various difficult surfaces
that exhibit interfering background luminescence when using conventional
luminescence techniques.
Cyanoacrylate fuming is probably the most important routine technique for
fingermark detection on non-porous surfaces. In the fingermark detection process,
the cyanoacrylate monomer forms a white fibrous layer of polycyanoacrylate on the
fingermark ridges. There are numerous holes in the fibrous polycyanoacrylate layer,
with an average diameter of 1–2 micrometres. Hence, it is worth investigating
smaller NaYF4:Er,Yb upconverter particles that can penetrate into the holes in the
polymer structure and remain trapped inside. In Chapter 5, three methods (sieving,
suspension and milling) were investigated to isolate the smaller particles from the
commercial NaYF4:Er,Yb powders. Owing to limitations with respect to
instrumentation and time, no ideal results were acquired.
Conventional upconverter materials are insoluble in water and other solvents, and
this limits their application when combined with cyanoacrylate fuming. The
possibility of making upconverters soluble or dispersible in water was investigated
by functionalizing them as nanoparticles with hydrophilic groups. Chapter 6 explores
the synthesis and use of four functionalised upconverters including UC-PEI
(NaYF4:Er,Yb/polyethylenimine), UC-AA (NaYF4:Er,Yb/azelaic acid), UC-PVP
(NaYF4:Er,Yb/polyvinyl pyrrolidone) and UC-AOT (NaYF4:Er,Yb /sodium bis(2-
ethylhexyl) sulfosuccinate) for staining CA-fumed fingermarks on various nonporous
surfaces. Among them, the UC-PEI and UC-AA showed strong luminescence
under 980 nm laser illumination, with the latter being more visually luminescent.
The UC-PEI and UC-AA showed some advantages for fingermark detection on
various difficult surfaces where background luminescence and printing interfered
with conventional luminescence enhancement. Long exposure times under a Rofin
Poliview system had to be employed in the imaging of fingermarks developed by the
functionalised upconverters. These long exposure times resulted in the substrate
itself being visible to some extent, which is different from the theoretical “ideal”
scenario that would provide bright fingermarks against a totally dark background.
However, functionalised upconverters still showed superior results to conventional
luminescence techniques for fingermark detection on some difficult substrates and
they have great potential to be improved through further research.
General discussion and conclusions are presented in Chapter 7. Possible future
directions for fingermark detection using upconverters are also presented.
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